The present invention relates to a focusing apparatus useful for recording/reproducing signals with respect to a digital versatile disc (hereinafter, referred simply to DVD), and a DVD optical disc apparatus using the focusing apparatus.
In recent years, techniques in digital signal processing, optical disc manufacture, and recording to optical discs have progressed rapidly. Lately, such progress has produced an optical disc when has a recording capacity of several times as much as a conventional compact disc (hereinafter, referred simply to CD) despite having a size the same as that of the aforesaid compact disc.
This optical disc of new type is generally called as DVD and is used to record and reproduce, by means of digital signals, information such as voice, images, programs, and computer-processible data. There is great expectations that DVD will be a general purpose recording medium of the future suitable for the multimedia times.
In the DVD field, there has been already established a worldwide standard (hereinafter, referred simply to DVD standards). According to the such DVD standards, a DVD has the same diameter as that of a CD, which is 120 mm, and has the same thickness as that of a CD, which is 1.2 mm. In order to increase a recording capacity, however, a DVD has a construction in which two discs having a thickness of 0.6 mm are put together so that recording/reproducing can be achieved using both sides of the resultant DVD disc. Meanwhile, a principal wavelength of a laser beam used for recording/reading is 650/635 nm; a Numerical Aperture (NA) of an objective lens used for recording/reading is 0.6; a track pitch is 0.74 .mu.m; the shortest pit length is 0.4 .mu.m; and the longest pit length is 2.13 .mu.m. Such a specification permits a DVD to have a record capacity of 4 giga bites or more on one side of the DVD.
As evident from the foregoing specification of DVD's the depth of a recording layer, namely a depth from the surface of a substrate to a reflective recording layer, is about 1/2 of the conventional CD. Further, the shortest pit length and the track pitch are about 1/2 of the CD as well. Moreover, the principal wavelength of a laser beam used for recording/reading is considerably smaller compared to that of 780 a CD which is nm. For this reason, it is very difficult to obtain a focus error signal using the same methods employed for obtaining a focus error signal of the conventional CD.
Specifically, in this type of optical disc, a signal is recorded on the reflective recording layer in the form of a pit. In order to read such a signal from the optical disc, a laser beam is usually radiated onto the rotating optical disc through an objective lens. The radiated light is reflected on the reflective recording layer formed on the optical disc, and, the reflected light is mostly detected, thereby obtaining a recording signal. Simultaneously, a focus error is detected by using part of the aforementioned reflected light, and focusing control for the objective lens is carried out so that the reflective recording layer can be situated in a focus position of the objective lens by using the detected error signal. Whether the of focusing control is good or bad depends upon optical conditions between the objective lens and the reflective recording layer, characteristics of the employed laser beam, etc.
In a DVD player known so far, the focusing control has been carried out according to the same methods employed in the conventional CD player. However, in such methods, a high-precise focusing control can not achieved in certain optical conditions. This causes a problem of lowering the S/N of the reproduction signal and jitter deterioration.